1. Field
This invention relates generally to the field of extruded plastic pipes and, in particular, to a method for incorporating a sealing system to connect such pipes.
2. Discussion of Related Art
PVC pipes have been used in delivering water to homes for decades. Easy installation, durability, and low prices have made it the most popular choice for contractors. An ASTM Standard published in 1993, describes the materials, requirements, and method of pressure rating PVC, and assigning to that product the material designation PVC-O, for oriented PVC. The Standard acknowledges that circumferential expansion of a PVC pipe results in orientation of the polymer molecules. This increases hoop tensile strength, which in the plastic pipe industry typically is measured by hydrostatic pressure rupture regression testing, Test procedures are given in the Standard for determining a categorized measure of the long-term hydrostatic strength of the PVC-O material (called Hydrostatic Design Basis). A pressure rating is assigned to the pipe itself based on wall thickness, diameter, and Hydrostatic Design Basis.
The Standard further indicates that a relationship exists between the expansion ratio and wall thickness ratio of the expanded pipe, and the long-term hydrostatic strength of the product. Wall thickness ratio refers to the ratio of the starting pipe wall thickness to that of the finished PVC-O pipe; expansion ratio is the ratio of the original to finished outside diameters. So, uniform expansion from a diameter of D to a diameter of 2D, with no change in material density, requires a thickness from T to T/2. The expansion ratio in this case is 0.5 and the wall thickness ration is 2.0.
Pipes formed from thermoplastic materials, including polyethylene, polypropylene and PVC, are used in a variety of industries. To form a joint between two sections of different pipes, the spigot or “male” pipe end is inserted into the socket or “female” pipe end. An annular, elastomeric sealing ring or gasket is typically seated within a groove formed in the socket end. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. To accommodate the aforementioned sealing ring, the socket will include an internal ring groove, typically formed by stretching the socket end over a uniquely shaped mandrel enlarged circumferentially to form an annular groove that will house the sealing ring.
The manufacture of the mating sections typically involves reformation by reheating and shaping the socket to the desired profile. The art of ebbing sockets (also called bells) on plastic pipes is well established, and there are various methods to do so. For example, a “Rieber” joint employs a combined mold element and sealing ring operable to seal a joint between a socket end and a spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, an elastomeric gasket is installed within a simultaneously formed internal groove in the socket end of a female pipe during a pipe belling process.
Nevertheless, the resonation of oriented materials can be problematical because the material will tend to revert if reheated. The oriented molecular structure, which in itself is created by a deformation process, will be lost. Furthermore, the deformation processes applied to the socket may alter the orientation level in such a way that the strength or other mechanical properties of the material are adversely affected.
As previously discussed, PVC pipes have been used in various water applications since the mid 1950s. Along with advancements in pipe construction, advancements have also been made in the sealing systems being utilized. Gaskets or “rings” that were once inserted by hand into a finished piece of pipe are now inserted by a machine as a “bell” is being formed. These flimsy pieces of rubber material have been replaced with rigid, sturdy, yet pliable material engineered to withstand hundreds of pounds of pressure through decades of service. In some instances, metal components are utilized along with the rubber gasket to create a virtually inseparable joint.
New materials are being utilized in the construction of PVC pipe, and with those new materials come new challenges in the manufacturing process. PVC-O, an example of a molecularly oriented pipe (“MOP”), presents different types of challenges due to its unusual construction. One of the most difficult challenges involves the “belling” process in which one end of the pipe is formed into a socket or female end, which in turn allows a spigot or male end to be inserted. The molecular properties of PVC-O make it extremely difficult to bell the pipe using current processes.
During the belling operation, the heated pipe end is forced over a forming mandrel, which typically has a sealing ring mounted about the mandrel. It is necessary to deform the heated pipe end as it passes over the forming mandrel to accommodate the sealing ring or other components. In some cases, the material of the MOP is already nearly stretched to its limit during pipe manufacture. When used in a Rieber belling process, the operation may alter the desired properties of the MOP or even cause the pipe to fail in certain cases.
Previously developed sealing gasket designs for PVC-O pipe have been shown to be effective sealing solutions for PVC-O in some instances, but they are unable to directly affect the scrap issue faced by manufacturers where MOP and particularly PVC-O pipe is not able to adequately withstand the stresses encountered during pipe-belling operations.